Known in the past has been a control system of an internal combustion engine comprising an exhaust purification catalyst provided in an exhaust passage of the internal combustion engine and an upstream side air-fuel ratio sensor provided at the upstream side of the exhaust purification catalyst in the direction of exhaust flow, wherein the control system is provided with a control part controlling an air-fuel ratio of the exhaust gas. In such a control system of an internal combustion engine, for example, the control part is used for feedback control of the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst so that the output air-fuel ratio of the upstream side air-fuel ratio sensor becomes a target air-fuel ratio. In addition, the target air-fuel ratio is alternately set to an air-fuel ratio richer than the stoichiometric air-fuel ratio (below, referred to as a “rich air-fuel ratio”) and a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio (below, referred to as a “lean air-fuel ratio”) (for example, PLT 1).
Further, in an internal combustion engine, when the pressure inside an intake port is higher than the pressure inside an exhaust port and valve overlap between an intake valve and an exhaust valve occurs, air is blown from the intake port through the inside of a cylinder to the exhaust port in a “scavenging” action. If scavenging occurs, the output air-fuel ratio of the upstream side air-fuel ratio sensor deviates to the rich side. Therefore, it has been proposed to estimate the ratio of the amount of scavenging to the amount of intake air (below, referred to as “the scavenging rate”) and correct the target air-fuel ratio by exactly the amount of deviation in the output air-fuel ratio of the upstream side air-fuel ratio sensor based on the estimated scavenging rate (for example, PLT 2).